The current theory of how the olfactory system distinguishes between odorant mixtures is based on the idea that the patterning of activity across many sensory neurons in the olfactory epithelium is integrated and recognized by higher centers in the brain. Odorant binding to membrane receptors on the cilia of olfactory receptor neurons activates one of several transduction pathways leading to depolarizing or hyperpolarizing responses that change the tonic firing rate of the olfactory receptor neurons. The discovery of multiple transduction pathways in a single olfactory receptor neuron suggests these cells are capable of integrating multiple inputs. Surprisingly, the biophysical and biochemical bases of olfactory mixture interactions have not been systematically investigated using both electrophysiological and fluorescent imaging methods. This proposal focuses on the mechanisms by which odorant mixtures activate different transduction pathways and thereby affect the integrated output of olfactory receptor neurons. The first specific aim is directed toward a characterization of the biophysical properties of the transduction pathways. This involves determination of concentration-response functions, conductance changes, and the ionic basis of transduction in squid olfactory neurons. The physiologically relevant odorants: dopamine, proline, AMP, and ADP will be used. Dopamine is an alarm substance found in squid ink that elicits strong aversive behavior in squid and hyperpolarizes olfactory receptor cells. Proline, AMP, and ADP stimulate appetitive behavior and depolarizing receptor potentials. The second specific aim will be the identification of the signal transduction mechanisms for each odorant. The third specific aim will be the effects of mixtures of odorants on activating multiple transduction pathways in a single receptor neuron. If mixture responses deviate from responses predicted on biophysical grounds, biochemical level interactions will be implicated and investigated.This work will provide insights into receptor level coding of odorant mixtures.